doc(bigtable): document connection pooling model (#9583)

google/cloud/bigtable/data_connection.h

@@ -45,6 +45,29 @@ GOOGLE_CLOUD_CPP_INLINE_NAMESPACE_BEGIN
  * To create a concrete instance, see `MakeDataConnection()`.
  *
  * For mocking, see `bigtable_mocks::MockDataConnection`.
+ *
+ * @par Connection Pool
+ * This class opens a number of [gRPC Channels] upon its construction. These
+ * channels each initiate a connection to the Cloud Bigtable service. This is a
+ * relatively slow operation that can take milliseconds, so applications are
+ * advised to reuse `DataConnection` objects when possible.
+ *
+ * The exact number of channels can be configured with the
+ * `google::cloud::GrpcNumChannelsOption`. If this option is not set, the class
+ * creates between 1 and 64 channels. The specific algorithm and number of
+ * channels is an implementation detail, and subject to change without notice.
+ *
+ * Each request sent from the client library cycles through the channels in the
+ * connection pool in a round robin fashion.
+ *
+ * Unused gRPC channels can enter an idle state. Refreshing them during a
+ * request may incur additional latency. To avoid this, the client library keeps
+ * connections open by refreshing the channels in a background thread. Advanced
+ * users can configure the frequency of the refreshes, or disable this feature,
+ * by using `bigtable::MinConnectionRefreshOption` and
+ * `bigtable::MaxConnectionRefreshOption`.
+ *
+ * [gRPC Channels]: https://grpc.io/docs/what-is-grpc/core-concepts/#channels
  */
 class DataConnection {
  public: